Ethylene and the Regulation of Physiological and Morphological Responses to Nutrient Deficiencies

Can nutrients act as signals under abiotic stress?

Plant cells are in constant communication to coordinate development processes and environmental reactions. Under stressful conditions, such communication allows the plant cells to adjust their activities and development. This is due to intercellular signaling events which involve several components. In plant development, cell-to-cell signaling is ensured by mobile signals hormones, hydrogen peroxide (H2O2), nitric oxide (NO), or hydrogen sulfide (H2S), as well as several transcription factors and small RNAs. Mineral nutrients, including macro and microelements, are determinant factors for plant growth and development and are, currently, recognized as potential signal molecules. This review aims to highlight the role of nutrients, particularly calcium, potassium, magnesium, nitrogen, phosphorus, and iron as signaling components with special attention to the mechanism of response against stress conditions.

Investigating the ecological implications of nanomaterials: Unveiling plants' notable responses to nano-pollution

The rapid advancement of nanotechnology has led to unprecedented innovations; however, it is crucial to analyze its environmental impacts carefully. This review thoroughly examines the complex relationship between plants and nanomaterials, highlighting their significant impact on ecological sustainability and ecosystem well-being. This study investigated the response of plants to nano-pollution stress, revealing the complex regulation of defense-related genes and proteins, and highlighting the sophisticated defense mechanisms in nature. Phytohormones play a crucial role in the complex molecular communication network that regulates plant responses to exposure to nanomaterials. The interaction between plants and nano-pollution influences plants' complex defense strategies. This reveals the interconnectedness of systems of nature. Nevertheless, these findings have implications beyond the plant domain. The incorporation of hyperaccumulator plants into pollution mitigation strategies has the potential to create more environmentally sustainable urban landscapes and improve overall environmental resilience. By utilizing these exceptional plants, we can create a future in which cities serve as centers of both innovation and ecological balance. Further investigation is necessary to explore the long-term presence of nanoparticles in the environment, their ability to induce genetic changes in plants over multiple generations, and their overall impact on ecosystems. In conclusion, this review summarizes significant scientific discoveries with broad implications beyond the confines of laboratories. This highlights the importance of understanding the interactions between plants and nanomaterials within the wider scope of environmental health. By considering these insights, we initiated a path towards the responsible utilization of nanomaterials, environmentally friendly management of pollution, and interdisciplinary exploration. We have the responsibility to balance scientific advancement and environmental preservation to create a sustainable future that combines nature's wisdom with human innovation.

Genome-wide identification of ETHYLENE INSENSITIVE 2 in Triticeae species reveals that TaEIN2-4D.1 regulates cadmium tolerance in Triticum aestivum

ETHYLENE INSENSITIVE 2 (EIN2), as the core component of the ethylene signaling pathway, can widely regulate plant growth, development, and stress responses. However, the comprehensive study and function of EIN2 in wheat Cadmium (Cd) stress remain largely unexplored. Here, we identified 33 EIN2 genes and designated as TaEIN2-2B to TaEIN2-Un.3 in Triticum aestivum. The analysis of cis-regulatory elements in promoter regions and RNA-Seq showed that TaEIN2s were functionally related to plant growth and development, as well as the response to biotic and abiotic stress. qRT-PCR analysis of TaEIN2s indicated their sensitivity to Cd stress. Compared with WT plants, TaEIN2-4D.1-RNAi transgenic wheat lines showed enhanced shoot and root elongation, dry weight and chlorophyll accumulation, together with a reduced accumulation of Cd in wheat grain. In addition, TaEIN2-4D.1-RNAi transgenic wheat lines showed enhanced Reactive Oxygen Species (ROS) scavenging capacity compared with WT plants. In conclusion, our research indicates that TaEIN2 plays a key role in response to cadmium stress in wheat, which provides valuable information for crop improvement.

Physiological and metabolic changes in response to Boron levels are mediated by ethylene affecting tomato fruit yield

Boron (B) is an essential nutrient for the plant, and its stress (both deficiency and toxicity) are major problems that affect crop production. Ethylene metabolism (both signaling and production) is important to plants' differently responding to nutrient availability. To better understand the connections between B and ethylene, here we investigate the function of ethylene in the responses of tomato (Solanum lycopersicum) plants to B stress (deficiency, 0 μM and toxicity, 640 μM), using ethylene related mutants, namely nonripening (nor), ripening-inhibitor (rin), never ripe (Nr), and epinastic (Epi). Our results show that B stress does not necessarily inhibit plant growth, but both B stress and ethylene signaling severely affected physiological parameters, such as photosynthesis, stomatal conductance, and chlorophyll a fluorescence. Under B toxicity, visible symptoms of toxicity appeared in the roots and margins of the older leaves through necrosis, caused by the accumulation of B which stimulated ethylene biosynthesis in the shoots. Both nor and rin (ethylene signaling) mutants presented similar responses, being these genotypes more sensitive and displaying several morphophysiological alterations, including fruit productivity reductions, in response to the B toxicity conditions. Therefore, our results suggest that physiological and metabolic changes in response to B fluctuations are likely mediated by ethylene signaling.

NO Is Not the Same as GSNO in the Regulation of Fe Deficiency Responses by Dicot Plants

Iron (Fe) is abundant in soils but with a poor availability for plants, especially in calcareous soils. To favor its acquisition, plants develop morphological and physiological responses, mainly in their roots, known as Fe deficiency responses. In dicot plants, the regulation of these responses is not totally known, but some hormones and signaling molecules, such as auxin, ethylene, glutathione (GSH), nitric oxide (NO) and S-nitrosoglutathione (GSNO), have been involved in their activation. Most of these substances, including auxin, ethylene, GSH and NO, increase their production in Fe-deficient roots while GSNO, derived from GSH and NO, decreases its content. This paradoxical result could be explained with the increased expression and activity in Fe-deficient roots of the GSNO reductase (GSNOR) enzyme, which decomposes GSNO to oxidized glutathione (GSSG) and NH3. The fact that NO content increases while GSNO decreases in Fe-deficient roots suggests that NO and GSNO do not play the same role in the regulation of Fe deficiency responses. This review is an update of the results supporting a role for NO, GSNO and GSNOR in the regulation of Fe deficiency responses. The possible roles of NO and GSNO are discussed by taking into account their mode of action through post-translational modifications, such as S-nitrosylation, and through their interactions with the hormones auxin and ethylene, directly related to the activation of morphological and physiological responses to Fe deficiency in dicot plants.

Responses of mangrove (Kandelia obovata) growth, photosynthesis, and rhizosphere soil properties to microplastic pollution

In this study, we used Kandelia obovata to explore the toxicities of three typical MPs in mangroves: polyethylene, polypropylene, and polyvinylchloride. MPs were mixed into soils at 5 % (w/w) for cultivation in mangrove tide-tank system. Plant growth and soil characteristics were determined after 12 months. The results showed that MPs multiply restricted root growth, leaf ionome, chlorophyll concentration, and photosynthetic efficiency, changed leaf photochemical efficiency and excited energy distribution. Polyethylene and polypropylene MPs with 150 μm sizes significantly reduced leaf and root biomass. As for soil properties, all MPs mainly reduced bulk density but improved porosity, with 600 μm polyethylene resulting in 1.4 g cm^-3 bulk density and 55.1 % porosity. This study represent the first paper highlighting that MPs negatively affect mangrove plants by affecting plant growth, leaf ionome, and photosynthesis, which may be related with the changed soil properties, especially reduced bulk density and improved porosity.

MtEIN2 affects nitrate uptake and accumulation of photosynthetic pigments under phosphate and nitrate deficiency in Medicago truncatula

Ethylene (ET) controls many facets of plant growth and development under abiotic and biotic stresses. MtEIN2, as a critical element of the ET signaling pathway, is essential in biotic interactions. However, the role of MtEIN2 in responding to abiotic stress, such as combined nutrient deficiency, is less known. To assess the role of ethylene signaling in nutrient uptake, we manipulated nitrate (NO₃ ^- ) and phosphate (Pi) availability for wild-type (WT) and the ethylene-insensitive (MtEIN2-defective) mutant, sickle, in Medicago truncatula. We measured leaf biomass and photosynthetic pigments in WT and sickle to identify conditions leading to different responses in both genotypes. Under combined NO₃ ^- and Pi deficiency, sickle plants had higher chlorophyll and carotenoid contents than WT plants. Under these conditions, nitrate content and gene expression levels of nitrate transporters were higher in the sickle mutant than in the WT. This led to the conclusion that MtEIN2 is associated with nitrate uptake and the content of photosynthetic pigments under combined Pi and NO₃ ^- deficiency in M. truncatula. We conclude that ethylene perception plays a critical role in regulating the nutrient status of plants.

Entomopathogenic Fungi-Mediated Solubilization and Induction of Fe Related Genes in Melon and Cucumber Plants

Endophytic insect pathogenic fungi have a multifunctional lifestyle; in addition to its well-known function as biocontrol agents, it may also help plants respond to other biotic and abiotic stresses, such as iron (Fe) deficiency. This study explores M. brunneum EAMa 01/58-Su strain attributes for Fe acquisition. Firstly, direct attributes include siderophore exudation (in vitro assay) and Fe content in shoots and in the substrate (in vivo assay) were evaluated for three strains of Beauveria bassiana and Metarhizium bruneum. The M. brunneum EAMa 01/58-Su strain showed a great ability to exudate iron siderophores (58.4% surface siderophores exudation) and provided higher Fe content in both dry matter and substrate compared to the control and was therefore selected for further research to unravel the possible induction of Fe deficiency responses, Ferric Reductase Activity (FRA), and relative expression of Fe acquisition genes by qRT-PCR in melon and cucumber plants.. In addition, root priming by M. brunneum EAMa 01/58-Su strain elicited Fe deficiency responses at transcriptional level. Our results show an early up-regulation (24, 48 or 72 h post inoculation) of the Fe acquisition genes FRO1, FRO2, IRT1, HA1, and FIT as well as the FRA. These results highlight the mechanisms involved in the Fe acquisition as mediated by IPF M. brunneum EAMa 01/58-Su strain.

The nonpathogenic strain of Fusarium oxysporum FO12 induces Fe deficiency responses in cucumber (Cucumis sativus L.) plants

MAIN CONCLUSION

FO12 strain enhances Fe deficiency responses in cucumber plants, probably through the production of ethylene and NO in the subapical regions of the roots. Rhizosphere microorganisms can elicit induced systemic resistance (ISR) in plants. This type of resistance involves complex mechanisms that confer protection to the plant against pathogen attack. Additionally, it has been reported by several studies that ISR and Fe deficiency responses are modulated by common pathways, involving some phytohormones and signaling molecules, like ethylene and nitric oxide (NO). The aim of this study was to determine whether the nonpathogenic strain of Fusarium oxysporum FO12 can induce Fe deficiency responses in cucumber (Cucumis sativus L.) plants. Our results demonstrate that the root inoculation of cucumber plants with the FO12 strain promotes plant growth after several days of cultivation, as well as rhizosphere acidification and enhancement of ferric reductase activity. Moreover, Fe-related genes, such as FRO1, IRT1 and HA1, are upregulated at certain times after FO12 inoculation either upon Fe-deficiency or Fe-sufficient conditions. Furthermore, it has been found that this fungus colonizes root cortical tissues, promoting the upregulation of ethylene synthesis genes and NO production in the root subapical regions. To better understand the effects of the FO12 strain on field conditions, cucumber plants were inoculated and cultivated in a calcareous soil under greenhouse conditions. The results obtained show a modification of some physiological parameters in the inoculated plants, such as flowering and reduction of tissue necrosis. Overall, the results suggest that the FO12 strain could have a great potential as a Fe biofertilizer and biostimulant.

Auxin efflux carrier ZmPIN1a modulates auxin reallocation involved in nitrate-mediated root formation

BACKGROUND

Auxin plays a crucial role in nitrate (NO₃^-)-mediated root architecture, and it is still unclear that if NO₃^- supply modulates auxin reallocation for regulating root formation in maize (Zea mays L.). This study was conducted to investigate the role of auxin efflux carrier ZmPIN1a in the root formation in response to NO₃^- supply.

RESULTS

Low NO₃^- (LN) promoted primary root (PR) elongation, while repressed the development of lateral root primordia (LRP) and total root length. LN modulated auxin levels and polar transport and regulated the expression of auxin-responsive and -signaling genes in roots. Moreover, LN up-regulated the expression level of ZmPIN1a, and overexpression of ZmPIN1a enhanced IAA efflux and accumulation in PR tip, while repressed IAA accumulation in LRP initiation zone, which consequently induced LN-mediated PR elongation and LR inhibition. The inhibition rate of PR length, LRP density and number of ZmPIN1a-OE plants was higher than that of wild-type plants after auxin transport inhibitor NPA treatment under NN and LN conditions, and the degree of inhibition of root growth in ZmPIN1a-OE plants was more obvious under LN condition.

CONCLUSION

These findings suggest that ZmPIN1a was involved in modulating auxin levels and transport to alter NO₃^--mediated root formation in maize.

Interplay between ethylene and nitrogen nutrition: How ethylene orchestrates nitrogen responses in plants

The stress hormone ethylene plays a key role in plant adaptation to adverse environmental conditions. Nitrogen (N) is the most quantitatively required mineral nutrient for plants, and its availability is a major determinant for crop production. Changes in N availability or N forms can alter ethylene biosynthesis and/or signaling. Ethylene serves as an important cellular signal to mediate root system architecture adaptation, N uptake and translocation, ammonium toxicity, anthocyanin accumulation, and premature senescence, thereby adapting plant growth and development to external N status. Here, we review the ethylene-mediated morphological and physiological responses and highlight how ethylene transduces the N signals to the adaptive responses. We specifically discuss the N-ethylene relations in rice, an important cereal crop in which ethylene is essential for its hypoxia survival.

Endophytic yeast protect plants against metal toxicity by inhibiting plant metal uptake through an ethylene-dependent mechanism

Toxic metal pollution requires significant adjustments in plant metabolism. Here, we show that the plant microbiota plays an important role in this process. The endophytic Sporobolomyces ruberrimus isolated from a serpentine population of Arabidopsis arenosa protected plants against excess metals. Coculture with its native host and Arabidopsis thaliana inhibited Fe and Ni uptake. It had no effect on host Zn and Cd uptake. Fe uptake inhibition was confirmed in wheat and rape. Our investigations show that, for the metal inhibitory effect, the interference of microorganisms in plant ethylene homeostasis is necessary. Application of an ethylene synthesis inhibitor, as well as loss-of-function mutations in canonical ethylene signalling genes, prevented metal uptake inhibition by the fungus. Coculture with S. ruberrimus significantly changed the expression of Fe homeostasis genes: IRT1, OPT3, OPT6, bHLH38 and bHLH39 in wild-type (WT) A. thaliana. The expression pattern of these genes in WT plants and in the ethylene signalling defective mutants significantly differed and coincided with the plant accumulation phenotype. Most notably, down-regulation of the expression of IRT1 solely in WT was necessary for the inhibition of metal uptake in plants. This study shows that microorganisms optimize plant Fe and Ni uptake by fine-tuning plant metal homeostasis.

Ethylene Signaling under Stressful Environments: Analyzing Collaborative Knowledge

Ethylene is a gaseous plant growth hormone that regulates various plant developmental processes, ranging from seed germination to senescence. The mechanisms underlying ethylene biosynthesis and signaling involve multistep mechanisms representing different control levels to regulate its production and response. Ethylene is an established phytohormone that displays various signaling processes under environmental stress in plants. Such environmental stresses trigger ethylene biosynthesis/action, which influences the growth and development of plants and opens new windows for future crop improvement. This review summarizes the current understanding of how environmental stress influences plants’ ethylene biosynthesis, signaling, and response. The review focuses on (a) ethylene biosynthesis and signaling in plants, (b) the influence of environmental stress on ethylene biosynthesis, (c) regulation of ethylene signaling for stress acclimation, (d) potential mechanisms underlying the ethylene-mediated stress tolerance in plants, and (e) summarizing ethylene formation under stress and its mechanism of action.

To grow or not to grow under nutrient scarcity: Target of rapamycin-ethylene is the question

To cope with nutrient scarcity, plants generally follow two main complementary strategies. On the one hand, they can slow down growing, mainly shoot growth, to diminish the demand of nutrients. We can call this strategy as "stop growing." On the other hand, plants can develop different physiological and morphological responses, mainly in their roots, aimed to facilitate the acquisition of nutrients. We can call this second strategy as "searching for nutrients." Both strategies are compatible and can function simultaneously but the interconnection between them is not yet well-known. In relation to the "stop growing" strategy, it is known that the TOR (Target Of Rapamycin) system is a central regulator of growth in response to nutrients in eukaryotic cells. TOR is a protein complex with kinase activity that promotes protein synthesis and growth while some SnRK (Sucrose non-fermenting 1-Related protein Kinases) and GCN (General Control Non-derepressible) kinases act antagonistically. It is also known that some SnRKs and GCNs are activated by nutrient deficiencies while TOR is active under nutrient sufficiency. In relation to the "searching for nutrients" strategy, it is known that the plant hormone ethylene participates in the activation of many nutrient deficiency responses. In this Mini Review, we discuss the possible role of ethylene as the hub connecting the "stop growing" strategy and the "searching for nutrients" strategy since very recent results also suggest a clear relationship of ethylene with the TOR system.

Transcriptome Level Analysis of Genes of Exogenous Ethylene Applied under Phosphorus Stress in Chinese Fir

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is a widely grown gymnosperm in China. Phosphorus (P) is an indispensable nutrient for the growth of Chinese fir. Inorganic phosphate (Pi) deficiency exists in soils of many Chinese fir planting area regions, and the trees themselves have limited efficiency in utilizing P from the soil. Ethylene is important in regulation responses to nutrient deficiencies. However, little is known about how ethylene signals participate in Pi stress in Chinese fir. A total of six different treatments were performed to reveal the transcript levels of Chinese fir under Pi, ethephon (an ethylene-releasing compound), and CoCl₂ (cobalt chloride, an ethylene biosynthesis inhibitor) treatments. We assembled a full-length reference transcriptome containing 22,243 unigenes as a reference for UMI RNA-seq (Digital RNA-seq). There were 586 Differentially Expressed Genes (DEGs) in the Pi starvation (NP) group, while DEGs from additional ethephon or CoCl₂ in NP were 708 and 292, respectively. Among the DEGs in each treatment, there were 83 TFs in these treatment groups. MYB (v-myb avian myeloblastosis viral oncogene homolog) family was the most abundant transcription factors (TFs). Three ERF (Ethylene response factor) family genes were identified when only ethylene content was imposed as a variable. Enrichment analysis indicated that the ascorbate and aldarate metabolism pathway plays a key role in resistance to Pi deficiency. This study provides insights for further elucidating the regulatory mechanism of Pi deficiency in Chinese fir.

Ethylene positively regulates Cd tolerance via reactive oxygen species scavenging and apoplastic transport barrier formation in rice

Ethylene regulates plant root growth and resistance to environment stress. However, the role and mechanism of ethylene signaling in response to Cd stress in rice remains unclear. Here, we revealed that ethylene signaling plays a positive role in the resistance of rice to Cd toxicity. Blocking the ethylene signal facilitated root elongation under normal conditions, but resulted in severe oxidative damage and inhibition of root growth under Cd stress. Conversely, ethylene signal enhancement by EIN2 overexpression caused root bending, similar to the response of roots to Cd stress, and displayed higher Cd tolerance than the wildtype (WT) plants. Comparative transcriptome analysis indicated EIN2-mediated upregulation of genes involved in flavonoid biosynthesis and peroxidase activity under Cd stress. The synthesis of phenolic acids and flavonoids were positively regulated by ethylene. Thus, the ein2 (ethylene insensitive 2) mutants displayed lower ROS scavenging capacity than the WT. Moreover, a significant increase in Cd accumulation and relatively increased apoplastic flow were observed in the root apex of the ein2 mutant compared with the WT plants. Overall, EIN2-mediated Cd resistance in rice is mediated by the upregulation of flavonoid biosynthesis and peroxidase activity to induce ROS scavenging, and apoplastic transport barrier formation reduces Cd uptake.

Strategies and Bottlenecks in Hexaploid Wheat to Mobilize Soil Iron to Grains

Our knowledge of iron (Fe) uptake and mobilization in plants is mainly based on Arabidopsis and rice. Although multiple players of Fe homeostasis have been elucidated, there is a significant gap in our understanding of crop species, such as wheat. It is, therefore, imperative not only to understand the different hurdles for Fe enrichment in tissues but also to address specifically the knowns/unknowns involved in the plausible mechanism of Fe sensing, signaling, transport, and subsequent storage in plants. In the present review, a unique perspective has been described in light of recent knowledge generated in wheat, an economically important crop. The strategies to boost efficient Fe uptake, transcriptional regulation, and long-distance mobilization in grains have been discussed, emphasizing recent biotechnological routes to load Fe in grains. This article also highlights the new elements of physiological and molecular genetics that underpin the mechanistic insight for the identified Fe-related genes and discusses the bottlenecks in unloading the Fe in grains. The information presented here will provide much-needed resources and directions to overcome challenges and design efficient strategies to enhance the Fe density in wheat grains.

Editing of the starch branching enzyme gene SBE2 generates high-amylose storage roots in cassava

The production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme gene SBE2 was firstly achieved. High-amylose cassava (Manihot esculenta Crantz) is desirable for starch industrial applications and production of healthier processed food for human consumption. In this study, we report the production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme 2 (SBE2). Mutations in two targeted exons of SBE2 were identified in all regenerated plants; these mutations, which included nucleotide insertions, and short or long deletions in the SBE2 gene, were classified into eight mutant lines. Three mutants, M6, M7 and M8, with long fragment deletions in the second exon of SBE2 showed no accumulation of SBE2 protein. After harvest from the field, significantly higher amylose (up to 56% in apparent amylose content) and resistant starch (up to 35%) was observed in these mutants compared with the wild type, leading to darker blue coloration of starch granules after quick iodine staining and altered starch viscosity with a higher pasting temperature and peak time. Further ¹H-NMR analysis revealed a significant reduction in the degree of starch branching, together with fewer short chains (degree of polymerization [DP] 15-25) and more long chains (DP>25 and especially DP>40) of amylopectin, which indicates that cassava SBE2 catalyzes short chain formation during amylopectin biosynthesis. Transition from A- to B-type crystallinity was also detected in the starches. Our study showed that CRISPR/Cas9-mediated mutagenesis of starch biosynthetic genes in cassava is an effective approach for generating novel varieties with valuable starch properties for food and industrial applications.

Overexpression of Lectin Receptor-Like Kinase 1 in Tomato Confers Resistance to Fusarium oxysporum f. sp. Radicis-Lycopersici

The disease Fusarium crown and root rot (FCRR), caused mainly by Fusarium oxysporum f. sp. radicis-lycopersici (FORL), seriously affects commercial tomato [Solanum lycopersicum (Sl)] yields. However, the genes that offer resistance to FORL are limited and the mechanism of resistance to FCRR is poorly understood. Lectin receptor-like kinases (LecRKs) play critical roles in defensive responses and immunity in many plant species; however, whether specific LecRKs are involved in the response of tomato plants to FORL is unclear. Here, we report that the expression of SlLecRK1/Solyc09g011070.1 was obviously induced by the infection of FORL. Biochemical and cell biological data revealed that SlLecRK1 is an active kinase that is located at the cell membrane, while real-time quantitative PCR data suggested that SlLecRK1 is mainly expressed in stems and roots. Genetic studies showed that overexpression of SlLecRK1 significantly improved the resistance of tomato plants to FORL but did not cause visible changes in plant growth and development compared with wild-type control plants. RNA-Seq data suggested that the positive effects of SlLecRK1 on the resistance of tomato plants to FORL occur mainly by triggering the expression of ethylene-responsive transcription factor (ERF) genes. Together, our findings not only identify a new target for the development of FCRR-resistant tomato varieties, they also demonstrate a molecular mechanism linking SlLecRK1 and ERFs in regulating the immune responses of tomato plants to FORL.

Assessment of Biofortification Approaches Used to Improve Micronutrient-Dense Plants That Are a Sustainable Solution to Combat Hidden Hunger

Malnutrition causes diseases, immune system disorders, deterioration in physical growth, mental development, and learning capacity worldwide. Micronutrient deficiency, known as hidden hunger, is a serious global problem. Biofortification is a cost-effective and sustainable agricultural strategy for increasing the concentrations or bioavailability of essential elements in the edible parts of plants, minimizing the risks of toxic metals, and thus reducing malnutrition. It has the advantage of delivering micronutrient-dense food crops to a large part of the global population, especially poor populations. Agronomic biofortification and biofertilization, traditional plant breeding, and optimized fertilizer applications are more globally accepted methods today; however, genetic biofortification based on genetic engineering such as increasing or manipulating (such as CRISPR-Cas9) the expression of genes that affect the regulation of metal homeostasis and carrier proteins that serve to increase the micronutrient content for higher nutrient concentration and greater productivity or that affect bioavailability is also seen as a promising high-potential strategy in solving this micronutrient deficiency problem. Data that micronutrients can help strengthen the immune system against the COVID-19 pandemic and other diseases has highlighted the importance of tackling micronutrient deficiencies. In this study, biofortification approaches such as plant breeding, agronomic techniques, microbial fertilization, and some genetic and nanotechnological methods used in the fight against micronutrient deficiency worldwide were compiled.

Coordinated Role of Nitric Oxide, Ethylene, Nitrogen, and Sulfur in Plant Salt Stress Tolerance

Salt stress significantly contributes to major losses in agricultural productivity worldwide. The sustainable approach for salinity-accrued toxicity has been explored. The use of plant growth regulators/phytohormones, mineral nutrients and other signaling molecules is one of the major approaches for reversing salt-induced toxicity in plants. Application of the signaling molecules such as nitric oxide (NO) and ethylene (ETH) and major mineral nutrient such as nitrogen (N) and sulfur (S) play significant roles in combatting the major consequences of salt stress impacts in plants. However, the literature available on gaseous signaling molecules (NO/ETH) or/and mineral nutrients (N/S) stands alone, and major insights into the role of NO or/and ETH along with N and S in plant-tolerance to salt remained unclear. Thus, this review aimed to (a) briefly overview salt stress and highlight salt-induced toxicity, (b) appraise the literature reporting potential mechanisms underlying the role of gaseous signaling molecules and mineral nutrient in salt stress tolerance, and (c) discuss NO and ETH along with N and S in relation to salt stress tolerance. In addition, significant issues that have still to be investigated in this context have been mentioned.

Ethylene and Nitric Oxide Involvement in the Regulation of Fe and P Deficiency Responses in Dicotyledonous Plants

Iron (Fe) and phosphorus (P) are two essential elements for plant growth. Both elements are abundant in soils but with poor availability for plants, which favor their acquisition by developing morphological and physiological responses in their roots. Although the regulation of the genes related to these responses is not totally known, ethylene (ET) and nitric oxide (NO) have been involved in the activation of both Fe-related and P-related genes. The common involvement of ET and NO suggests that they must act in conjunction with other specific signals, more closely related to each deficiency. Among the specific signals involved in the regulation of Fe- or P-related genes have been proposed Fe-peptides (or Fe ion itself) and microRNAs, like miR399 (P), moving through the phloem. These Fe- or P-related phloem signals could interact with ET/NO and confer specificity to the responses to each deficiency, avoiding the induction of the specific responses when ET/NO increase due to other nutrient deficiencies or stresses. Besides the specificity conferred by these signals, ET itself could confer specificity to the responses to Fe- or P-deficiency by acting through different signaling pathways in each case. Given the above considerations, there are preliminary results suggesting that ET could regulate different nutrient responses by acting both in conjunction with other signals and through different signaling pathways. Because of the close relationship among these two elements, a better knowledge of the physiological and molecular basis of their interaction is necessary to improve their nutrition and to avoid the problems associated with their misuse. As examples of this interaction, it is known that Fe chlorosis can be induced, under certain circumstances, by a P over- fertilization. On the other hand, Fe oxides can have a role in the immobilization of P in soils. Qualitative and quantitative assessment of the dynamic of known Fe- and P-related genes expression, selected ad hoc and involved in each of these deficiencies, would allow us to get a profound knowledge of the processes that regulate the responses to both deficiencies. The better knowledge of the regulation by ET of the responses to these deficiencies is necessary to properly understand the interactions between Fe and P. This will allow the obtention of more efficient varieties in the absorption of P and Fe, and the use of more rational management techniques for P and Fe fertilization. This will contribute to minimize the environmental impacts caused by the use of P and Fe fertilizers (Fe chelates) in agriculture and to adjust the costs for farmers, due to the high prices and/or scarcity of Fe and P fertilizers. This review aims to summarize the latest advances in the knowledge about Fe and P deficiency responses, analyzing the similarities and differences among them and considering the interactions among their main regulators, including some hormones (ethylene) and signaling substances (NO and GSNO) as well as other P- and Fe-related signals.

Comparative Study of Several Fe Deficiency Responses in the Arabidopsis thaliana Ethylene Insensitive Mutants ein2-1 and ein2-5

Iron (Fe) is an essential micronutrient for plants since it participates in essential processes such as photosynthesis, respiration and nitrogen assimilation. Fe is an abundant element in most soils, but its availability for plants is low, especially in calcareous soils. Fe deficiency causes Fe chlorosis, which can affect the productivity of the affected crops. Plants favor Fe acquisition by developing morphological and physiological responses in their roots. Ethylene (ET) and nitric oxide (NO) have been involved in the induction of Fe deficiency responses in dicot (Strategy I) plants, such as Arabidopsis. In this work, we have conducted a comparative study on the development of subapical root hairs, of the expression of the main Fe acquisition genes FRO2 and IRT1, and of the master transcription factor FIT, in two Arabidopsis thaliana ET insensitive mutants, ein2-1 and ein2-5, affected in EIN2, a critical component of the ET transduction pathway. The results obtained show that both mutants do not induce subapical root hairs either under Fe deficiency or upon treatments with the ET precursor 1-aminocyclopropane-1-carboxylate (ACC) and the NO donor S-nitrosoglutathione (GSNO). By contrast, both of them upregulate the Fe acquisition genes FRO2 and IRT1 (and FIT) under Fe deficiency. However, the upregulation was different when the mutants were exposed to ET [ACC and cobalt (Co), an ET synthesis inhibitor] and GSNO treatments. All these results clearly support the participation of ET and NO, through EIN2, in the regulation of subapical root hairs and Fe acquisition genes. The results will be discussed, taking into account the role of both ET and NO in the regulation of Fe deficiency responses.

Influence of Ethylene Signaling in the Crosstalk Between Fe, S, and P Deficiency Responses in Arabidopsis thaliana

To cope with P, S, or Fe deficiency, dicot plants, like Arabidopsis, develop several responses (mainly in their roots) aimed to facilitate the mobilization and uptake of the deficient nutrient. Within these responses are the modification of root morphology, an increased number of transporters, augmented synthesis-release of nutrient solubilizing compounds and the enhancement of some enzymatic activities, like ferric reductase activity (FRA) or phosphatase activity (PA). Once a nutrient has been acquired in enough quantity, these responses should be switched off to minimize energy costs and toxicity. This implies that they are tightly regulated. Although the responses to each deficiency are induced in a rather specific manner, crosstalk between them is frequent and in such a way that P, S, or Fe deficiency can induce responses related to the other two nutrients. The regulation of the responses is not totally known but some hormones and signaling substances have been involved, either as activators [ethylene (ET), auxin, nitric oxide (NO)], or repressors [cytokinins (CKs)]. The plant hormone ET is involved in the regulation of responses to P, S, or Fe deficiency, and this could partly explain the crosstalk between them. In spite of these crosslinks, it can be hypothesized that, to confer the maximum specificity to the responses of each deficiency, ET should act in conjunction with other signals and/or through different transduction pathways. To study this latter possibility, several responses to P, S, or Fe deficiency have been studied in the Arabidopis wild-type cultivar (WT) Columbia and in some of its ethylene signaling mutants (ctr1, ein2-1, ein3eil1) subjected to the three deficiencies. Results show that key elements of the ET transduction pathway, like CTR1, EIN2, and EIN3/EIL1, can play a role in the crosstalk among nutrient deficiency responses.

Physiological response of cucumber (Cucumis sativus L.) leaves to polystyrene nanoplastics pollution

Microplastics pollution in farmlands has become a major concern. However, few studies have assessed the effects of microplastics on higher plants. In this study, we investigated the influence of polystyrene nanoplastics (PSNPs, 50 mg L^-1), with four different particle sizes (100, 300, 500, and 700 nm), on the physiological and biochemical indexes of cucumber leaves. The biomass of cucumber plants significantly decreased after exposure to 300 nm PSNPs. Similarly, the chlorophyll a, chlorophyll b, soluble sugar, carotenoid, and proline content, as well as the fluorescence of cucumber leaves were significantly reduced by 100 nm PSNPs. Malondialdehyde, proline, peroxidase gene expression and enzyme activity, and hydrogen peroxide content significantly increased in cucumber leaves exposed to 700 nm PSNPs. In addition, increasing PSNPs particle size led to decreased relative expression levels and activities of the major antioxidant enzymes superoxide dismutase and catalase, while vitamin C and soluble protein content significantly increased. Overall, our results indicated that PSNPs affect the photosynthetic, antioxidant, and sugar metabolism systems of cucumber leaves, with the latter clearly affecting the total biomass of cucumber plants. The benzene ring resulting from the degradation of PSNPs in cucumber leaves may be the main factor affecting chlorophyll metabolism and sugar metabolism. Our findings provide a scientific basis for the risk assessment of PSNPs exposure in soil-plant systems.

Metabolomic Analyses Provide New Insights into Signaling Mechanisms for Nutrient Uptake by Lateral Roots of Pruned Tea Plant (Camellia sinensis)

Pruning is an important plant management practice in tea cultivation. However, the mechanism underlying the dynamics of nutrient uptake by roots of pruned tea is unknown. This study investigated the metabolic alterations in lateral roots of pruned tea to unveil the mechanism of nutrient uptake. Elemental analysis revealed that pruning significantly increases the uptake of nutrients by lateral roots. Metabolic profiling showed significant metabolic variations in lateral roots of pruned tea. Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis revealed that flavonoid biosynthesis, phenylpropanoid biosynthesis, and amino acid metabolism were differentially regulated in lateral roots. Caffeine metabolism was significantly hindered, while ethylene signaling was significantly induced in lateral roots of pruned plants. In addition, intermediates in the tricarboxylic acid (TCA) cycle were upregulated, indicating high rates of the TCA cycle. Therefore, pathways related to phenylpropanoid biosynthesis, TCA cycle, ethylene biosynthesis, and metabolism of amino acids contribute to higher nutrient uptake by lateral roots of the tea plant.

Crosstalk between heterotrimeric G protein-coupled signaling pathways and WRKY transcription factors modulating plant responses to suboptimal micronutrient conditions

Nutrient stresses induce foliar chlorosis and growth defects. Here we propose heterotrimeric G proteins as signaling mediators of various nutrient stresses, through meta-analyses of >20 transcriptomic data sets associated with nutrient stresses or G protein mutations. Systematic comparison of transcriptomic data yielded 104 genes regulated by G protein subunits under common nutrient stresses: 69 genes under Gβ subunit (AGB1) control and 35 genes under Gα subunit (GPA1) control. Quantitative real-time PCR experiments validate that several transcription factors and metal transporters changed in expression level under suboptimal iron, zinc, and/or copper concentrations, while being misregulated in the Arabidopsis Gβ-null (agb1) mutant. The agb1 mutant had altered metal ion profiles and exhibited severe growth arrest under zinc stress, and aberrant root waving under iron and zinc stresses, while the Gα-null mutation attenuated leaf chlorosis under iron deficiency in both Arabidopsis and rice. Our transcriptional network analysis inferred computationally that WRKY-family transcription factors mediate the AGB1-dependent nutrient responses. As corroborating evidence of our inference, ectopic expression of WRKY25 or WRKY33 rescued the zinc stress phenotypes and the expression of zinc transporters in the agb1-2 background. These results, together with Gene Ontology analyses, suggest two contrasting roles for G protein-coupled signaling pathways in micronutrient stress responses: one enhancing general stress tolerance and the other modulating ion homeostasis through WRKY transcriptional regulatory networks. In addition, tolerance to iron stress in the rice Gα mutant provides an inroad to improve nutrient stress tolerance of agricultural crops by manipulating G protein signaling.

Target of Rapamycin (TOR) Negatively Regulates Ethylene Signals in Arabidopsis

Target of rapamycin (TOR) acts as a master regulator in coordination of cell growth with energy and nutrient availability. Despite the increased appreciation of the essential role of the TOR complex in interaction with phytohormone signaling, little is known about its function on ethylene signaling. Here, through expression analysis, genetic and biochemical approaches, we reveal that TOR functions in the regulation of ethylene signals. Transcriptional analysis indicates that TOR inhibition by AZD8055 upregulated senescence- and ethylene-related genes expression. Furthermore, ethylene insensitive mutants like etr1-1, ein2-5 and ein3 eil1, showed more hyposensitivity to AZD8055 than that of WT in hypocotyl growth inhibition. Similarly, blocking ethylene signals by ethylene action inhibitor Ag+ or biosynthesis inhibitor aminoethoxyvinylglycine (AVG) largely rescued hypocotyl growth even in presence of AZD8055. In addition, we also demonstrated that Type 2A phosphatase-associated protein of 46 kDa (TAP46), a downstream component of TOR signaling, physically interacts with 1-aminocy-clopropane-1-carboxylate (ACC) synthase ACS2 and ACS6. Arabidopsis overexpressing ACS2 or ACS6 showed more hypersensitivity to AZD8055 than WT in hypocotyl growth inhibition. Moreover, ACS2/ACS6 protein was accumulated under TOR suppression, implying TOR modulates ACC synthase protein levels. Taken together, our results indicate that TOR participates in negatively modulating ethylene signals and the molecular mechanism is likely involved in the regulation of ethylene biosynthesis by affecting ACSs in transcription and protein levels.

The physiological mechanism underlying root elongation in response to nitrogen deficiency in crop plants

In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

Emerging concepts of potassium homeostasis in plants

Potassium (K+) is an essential cation in all organisms that influences crop production and ecosystem stability. Although most soils are rich in K minerals, relatively little K+ is present in forms that are available to plants. Moreover, leaching and run-off from the upper soil layers contribute to K+ deficiencies in agricultural soils. Hence, the demand for K fertilizer is increasing worldwide. K+ regulates multiple processes in cells and organs, with K+ deficiency resulting in decreased plant growth and productivity. Here, we discuss the complexity of the reactive oxygen species-calcium-hormone signalling network that is responsible for the sensing of K+ deficiency in plants, together with genetic approaches using K+ transporters that have been used to increase K+ use efficiency (KUE) in plants, particularly under environmental stress conditions such as salinity and heavy metal contamination. Publicly available rice transcriptome data are used to demonstrate the two-way relationship between K+ and nitrogen nutrition, highlighting how each nutrient can regulate the uptake and root to shoot translocation of the other. Future research directions are discussed in terms of this relationship, as well as prospects for molecular approaches for the generation of improved varieties and the implementation of new agronomic practices. An increased knowledge of the systems that sense and take up K+, and their regulation, will not only improve current understanding of plant K+ homeostasis but also facilitate new research and the implementation of measures to improve plant KUE for sustainable food production.

Ethylene and Phloem Signals Are Involved in the Regulation of Responses to Fe and P Deficiencies in Roots of Strategy I Plants

Iron (Fe) and phosphorus (P) are two essential mineral nutrients whose acquisition by plants presents important environmental and economic implications. Both elements are abundant in most soils but scarcely available to plants. To prevent Fe or P deficiency dicot plants initiate morphological and physiological responses in their roots aimed to specifically acquire these elements. The existence of common signals in Fe and P deficiency pathways suggests the signaling factors must act in conjunction with distinct nutrient-specific signals in order to confer tolerance to each deficiency. Previous works have shown the existence of cross talk between responses to Fe and P deficiency, but details of the associated signaling pathways remain unclear. Herein, the impact of foliar application of either P or Fe on P and Fe responses was studied in P- or Fe-deficient plants of Arabidopsis thaliana, including mutants exhibiting altered Fe or P homeostasis. Ferric reductase and acid phosphatase activities in roots were determined as well as the expression of genes related to P and Fe acquisition. The results obtained showed that Fe deficiency induces the expression of P acquisition genes and phosphatase activity, whereas P deficiency induces the expression of Fe acquisition genes and ferric reductase activity, although only transitorily. Importantly, these responses were reversed upon foliar application of either Fe or P on nutrient-starved plants. Taken together, the results reveal interactions between P- and Fe-related phloem signals originating in the shoots that likely interact with hormones in the roots to initiate adaptive mechanisms to tolerate deficiency of each nutrient.

Ammonium regulates Fe deficiency responses by enhancing nitric oxide signaling in Arabidopsis thaliana

The accumulation of NH₄⁺ in response to Fe deficiency plays a role not only in the remobilization of Fe from the root cell wall, but also in the transportation of Fe from root to shoot. Ammonium (NH₄⁺) plays an important role in phosphorus-deficiency responses in rice, but its role in responses to Fe deficiency remains unknown. Here, we demonstrate that the accumulation of NH₄⁺ plays a pivotal role when Arabidopsis thaliana plants are subject to Fe deficiency. The Arabidopsis amt1-3 mutant, which is defective in endogenous NH₄⁺ sensing, exhibited increased sensitivity to Fe deficiency compared to WT (wild type; Col-0). In addition, exogenous application of NH₄⁺ significantly alleviated Fe deficiency symptoms in plants. NH₄⁺ triggers the production of nitric oxide (NO), which then induces ferric-chelate reductase (FCR) activity and accelerates the release of Fe from the cell wall, especially hemicellulose, thereby increasing the availability of soluble Fe in roots. NH₄⁺ also increases soluble Fe levels in shoots by upregulating genes involved in Fe translocation, such as FRD3 (FERRIC REDUCTASE DEFECTIVE3) and NAS1 (NICOTIANAMINE SYNTHASE1), hence, alleviating leaf chlorosis. Overall, NH₄⁺ plays an important role in the reutilization of Fe from the cell wall and the redistribution of Fe from root to shoot in Fe-deficient Arabidopsis, a process dependent on NO accumulation.

Applications of metabolomics in assessing ecological effects of emerging contaminants and pollutants on plants

Metabolomics, the global profiling of metabolite composition, is a powerful technique that can be applied to answer a diverse set of research questions concerning effects of toxicants on organisms. It has recently emerged as a tool to understand complex environmental perturbations in biological systems, especially at sub-lethal concentrations. Organisms can be affected by different stressors such as xenobiotics or increase in concentration of natural compounds such as nitrogen, phosphorous, and sulfur. Metabolomics has facilitated a better understanding of the effects of these perturbations on organisms such as plants, animals, and humans providing phenotypic and biological information in a high throughput manner. In this review, we will discuss recent applications of metabolomics to study the ecological effects of different environmental perturbations, including nanoparticles, pharmaceuticals and personal care products, pesticides, as well as the changes in natural compounds found in the environment with a focus on plant systems.

A metabolomics-based approach for the evaluation of off-tree ripening conditions and different postharvest treatments in mangosteen (Garcinia mangostana)

INTRODUCTION

Metabolomics is an important tool to support postharvest fruit development and ripening studies. Mangosteen (Garcinia mangostana L.) is a tropical fruit with high market value but has short shelf-life during postharvest handling. Several postharvest technologies have been applied to maintain mangosteen fruit quality during storage. However, there is no study to evaluate the metabolite changes that occur in different harvesting and ripening condition. Additionally, the effect of postharvest treatment using a metabolomics approach has never been studied in mangosteen.

OBJECTIVES

The aims of this study were to evaluate the metabolic changes between different harvesting and ripening condition and to evaluate the effect of postharvest treatment in mangosteen.

METHODS

Mangosteen ripening stage were collected with several different conditions ("natural on-tree", "random on-tree" and "off-tree"). The metabolite changes were investigated for each ripening condition. Additionally, mangosteen fruit was harvested in stage 2 and was treated with several different treatments (storage at low temperature (LT; 12.3 ± 1.4 °C) and stress inducer treatment (methyl jasmonate and salicylic acid) in comparison with control treatment (normal temperature storage) and the metabolite changes were monitored over the course of 10 days after treatment. The metabolome data obtained from gas chromatography coupled with mass spectrometry were analyzed by multivariate analysis, including hierarchical clustering analysis, principal component analysis, and partial to latent squares analysis.

RESULTS

"On-tree" ripening condition showed the progression of ripening process in accordance with the accumulation of some aroma precursor metabolites in the flesh part and pectin breakdown in the peel part. Interestingly, similar trend was found in the "off-tree" ripening condition although the progression of ripening process observed through color changes occurred much faster compared to "on-tree" ripening. Additionally, low-temperature treatment is shown as the most effective treatment to prolong mangosteen shelf-life among all postharvest treatments tested in this study compared to control treatment. After postharvest treatment, a total of 71 and 65 metabolites were annotated in peel and flesh part of mangosteen, respectively. Several contributed metabolites (xylose, galactose, galacturonic acid, glucuronate, glycine, and rhamnose) were decreased after treatment in the peel part. However, low-temperature treatment did not show any significant differences compared to a room temperature treatment in the flesh part.

CONCLUSIONS

Our findings clearly indicate that there is a similar trend of metabolic changes between on-tree and off-tree ripening conditions. Additionally, postharvest treatment directly or indirectly influences many metabolic processes (cell-wall degrading process, sweet-acidic taste quality) during postharvest treatment.

CHITINASE LIKE1 Regulates Root Development of Dark-Grown Seedlings by Modulating Ethylene Biosynthesis in Arabidopsis thaliana

The plant hormone ethylene plays a regulatory role in development in light- and dark-grown seedlings. We previously isolated a group of small-molecule compounds with a quinazolinone backbone, which were named acsinones (for ACC synthase inhibitor quinazolinones), that act as uncompetitive inhibitors of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS). Thus, the triple response phenotype, which consists of shortened hypocotyls and roots, radial swelling of hypocotyls and exaggerated curvature of apical hooks, was suppressed by acsinones in dark-grown (etiolated) ethylene overproducer (eto) seedlings. Here, we describe our isolation and characterization of an Arabidopsis revert to eto1 9 (ret9) mutant, which showed reduced sensitivity to acsinones in etiolated eto1 seedlings. Map-based cloning of RET9 revealed an amino acid substitution in CHITINASE LIKE1 (CTL1), which is required for cell wall biogenesis and stress resistance in Arabidopsis. Etiolated seedlings of ctl1^ret9 showed short hypocotyls and roots, which were augmented in combination with eto1-4. Consistently, ctl1^ret9 seedlings showed enhanced sensitivity to exogenous ACC to suppress primary root elongation as compared with the wild type. After introducing ctl1^ret9 to mutants completely insensitive to ethylene, genetic analysis indicated that an intact ethylene response pathway is essential for the alterations in root and apical hook but not hypocotyl in etiolated ctl1^ret9 seedlings. Furthermore, a mild yet significantly increased ethylene level in ctl1 mutants was related to elevated mRNA level and activity of ACC oxidase (ACO). Moreover, genes associated with ethylene biosynthesis (ACO1 and ACO2) and response (ERF1 and EDF1) were upregulated in etiolated ctl1^ret9 seedlings. By characterizing a new recessive allele of CTL1, we reveal that CTL1 negatively regulates ACO activity and the ethylene response, which thus contributes to understanding a role for ethylene in root elongation in response to perturbed cell wall integrity.

A Functional Connection between the Circadian Clock and Hormonal Timing in Arabidopsis

The rotation of the Earth entails changes in environmental conditions that pervasively influence an organism's physiology and metabolism. An internal cellular mechanism known as the circadian clock acts as an internal timekeeper that is able to perceive the changes in environmental cues to generate 24-h rhythms in synchronization with daily and seasonal fluctuations. In plants, the circadian clock function is particularly important and regulates nearly every aspect of plant growth and development as well as proper responses to stresses. The circadian clock does not function in isolation but rather interconnects with an intricate network of different pathways, including those of phytohormones. Here, we describe the interplay of the circadian clock with a subset of hormones in Arabidopsis. The molecular components directly connecting the circadian and hormone pathways are described, highlighting the biological significance of such connections in the control of growth, development, fitness, and survival. We focus on the overlapping as well as contrasting circadian and hormonal functions that together provide a glimpse on how the Arabidopsis circadian system regulates hormone function in response to endogenous and exogenous cues. Examples of feedback regulation from hormone signaling to the clock are also discussed.

A Shoot Fe Signaling Pathway Requiring the OPT3 Transporter Controls GSNO Reductase and Ethylene in Arabidopsis thaliana Roots

Ethylene, nitric oxide (NO) and glutathione (GSH) increase in Fe-deficient roots of Strategy I species where they participate in the up-regulation of Fe acquisition genes. However, S-nitrosoglutathione (GSNO), derived from NO and GSH, decreases in Fe-deficient roots. GSNO content is regulated by the GSNO-degrading enzyme S-nitrosoglutathione reductase (GSNOR). On the other hand, there are several results showing that the regulation of Fe acquisition genes does not solely depend on hormones and signaling molecules (such as ethylene or NO), which would act as activators, but also on the internal Fe content of plants, which would act as a repressor. Moreover, different results suggest that total Fe in roots is not the repressor of Fe acquisition genes, but rather the repressor is a Fe signal that moves from shoots to roots through the phloem [hereafter named LOng Distance Iron Signal (LODIS)]. To look further in the possible interactions between LODIS, ethylene and GSNOR, we compared Arabidopsis WT Columbia and LODIS-deficient mutant opt3-2 plants subjected to different Fe treatments that alter LODIS content. The opt3-2 mutant is impaired in the loading of shoot Fe into the phloem and presents constitutive expression of Fe acquisition genes. In roots of both Columbia and opt3-2 plants we determined 1-aminocyclopropane-1-carboxylic acid (ACC, ethylene precursor), expression of ethylene synthesis and signaling genes, and GSNOR expression and activity. The results obtained showed that both 'ethylene' (ACC and the expression of ethylene synthesis and signaling genes) and 'GSNOR' (expression and activity) increased in Fe-deficient WT Columbia roots. Additionally, Fe-sufficient opt3-2 roots had higher 'ethylene' and 'GSNOR' than Fe-sufficient WT Columbia roots. The increase of both 'ethylene' and 'GSNOR' was not related to the total root Fe content but to the absence of a Fe shoot signal (LODIS), and was associated with the up-regulation of Fe acquisition genes. The possible relationship between GSNOR(GSNO) and ethylene is discussed.

Similarities and Differences in the Acquisition of Fe and P by Dicot Plants

This review deals with two essential plant mineral nutrients, iron (Fe) and phosphorus (P); the acquisition of both has important environmental and economic implications. Both elements are abundant in soils but are scarcely available to plants. To prevent deficiency, dicot plants develop physiological and morphological responses in their roots to specifically acquire Fe or P. Hormones and signalling substances, like ethylene, auxin and nitric oxide (NO), are involved in the activation of nutrient-deficiency responses. The existence of common inducers suggests that they must act in conjunction with nutrient-specific signals in order to develop nutrient-specific deficiency responses. There is evidence suggesting that P- or Fe-related phloem signals could interact with ethylene and NO to confer specificity to the responses to Fe- or P-deficiency, avoiding their induction when ethylene and NO increase due to other nutrient deficiency or stress. The mechanisms responsible for such interaction are not clearly determined, and thus, the regulatory networks that allow or prevent cross talk between P and Fe deficiency responses remain obscure. Here, fragmented information is drawn together to provide a clearer overview of the mechanisms and molecular players involved in the regulation of the responses to Fe or P deficiency and their interactions.

Ethylene signaling cross-talk with other hormones in Arabidopsis thaliana exposed to contrasting phosphate availability: Differential effects in roots, leaves and fruits

Ethylene signaling plays a major role in the regulation of plant growth, but its cross-talk with other phytohormones is still poorly understood. Here, we investigated whether or not a defect in ethylene signaling, particularly in the ETHYLENE INSENSITIVE3 (EIN3) transcription factor, alters plant growth and influences the contents of other phytohormones. With this aim, a hormonal profiling approach using ultrahigh performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) was used to unravel organ-specific responses (in roots, leaves and fruits) in the ein3-1 mutant and wild-type A. thaliana plants exposed to contrasting phosphate (Pi) availability. A defect in ethylene signaling in the ein3-1 mutant increased the biomass of roots, leaves and fruits, both at 0.5 mM and 1 mM Pi, thus indicating the growth-inhibitory role of ethylene in all tested organs. The hormonal profiling in roots revealed a cross-talk between ethylene signaling and other phytohormones, as indicated by increases in the contents of auxin, gibberellins and the stress-related hormones, abscisic acid, salicylic acid and jasmonic acid. The ein3-1 mutant also showed increased cytokinin contents in leaves. Reduced Pi availability (from 1 mM to 0.5 mM Pi) affected fruit growth, but not root and leaf growth, thus indicating mild Pi deficiency. It is concluded that ethylene signaling plays a major role in the modulation of plant growth in A. thaliana and that the ein3-1 mutant is not only altered in ethylene signaling but in the contents of several phytohormones in an organ-specific manner, thus indicating a hormonal cross-talk.

Pleiotropic effects of enhancing vacuolar K/H exchange in tomato

Cation antiporters of the NHX family are widely regarded as determinants of salt tolerance due to their capacity to drive sodium (Na) and sequester it into vacuoles. Recent work shows, however, that NHX transporters are primarily involved in vacuolar potassium (K) storage. Over-expression of the K/H antiporter AtNHX1 in tomato increases K accumulation into vacuoles and plant sensitivity to K deprivation. Here we show that the appearance of early leaf symptoms of K deficiency was associated with higher concentration of polyamines. Transgenic roots exhibited a greater sensitivity than shoots to K deprivation with changes in the composition of the free amino acids pool, total sugars and organic acids. Concentrations of amides (glutamine), amino acids (arginine) and sugars significantly increased in root, together with a reduction in malate and succinate concentrations. The concentration of pyruvate and the activity of pyruvate kinase were greater in the transgenic roots before K withdrawal although both parameters were depressed by K deprivation and approached wild-type levels. In the longer term, the over-expression of the NHX1 antiporter affected root growth and biomass partitioning (shoot/root ratio). Greater ethylene release produced longer stem internodes and leaf curling in the transgenic line. Our data show that enhanced sequestration of K by the NHX antiporter in the vacuoles altered cellular K homeostasis and had deeper physiological consequences than expected. Early metabolic changes lead later on to profound morphological and physiological adjustments resulting eventually in the loss of nutrient use efficiency.

High-throughput sequencing analysis revealed the regulation patterns of small RNAs on the development of A. comosus var. bracteatus leaves

Studies of the molecular mechanisms involved in the formation of the albino leaf cells are important for understanding the development of chimera leaves in Ananas comosus var. bracteatus. In this study, we identified a total of 163 novel miRNAs involved in the development of complete white (CWh) and complete green (CGr) leaves using high-throughput sequencing method. The potential miRNA target genes were predicted and annotated using the NR, Swiss-Prot, GO, COG, KEGG, KOG and Pfam databases. The main biological processes regulated by miRNAs were revealed. The miRNAs which potentially play important roles in the development of the leaves and the albino of the CWh leaf cells were selected and their expression patterns were analyzed. The expression levels of nine miRNAs and their potential target genes were studied using qRT-PCR. These results will help to elucidate the functional and regulatory roles of miRNAs in the formation of the albino cells and the development of the leaves of A. comosus var. bracteatus. These data may also be helpful as a resource for studies of small RNA in other leaf color chimeric plant species.

Molecular regulation of aluminum resistance and sulfur nutrition during root growth

Aluminum toxicity and sulfate deprivation both regulate microRNA395 expression, repressing its low-affinity sulfate transporter ( SULTR2;1 ) target. Sulfate deprivation also induces the high-affinity sulfate transporter gene ( SULTR12 ), allowing enhanced sulfate uptake. Few studies about the relationships between sulfate, a plant nutrient, and aluminum, a toxic ion, are available; hence, the molecular and physiological processes underpinning this interaction are poorly understood. The Al-sulfate interaction occurs in acidic soils, whereby relatively high concentrations of trivalent toxic aluminum (Al³⁺) may hamper root growth, limiting uptake of nutrients, including sulfur (S). On the other side, Al³⁺ may be detoxified by complexation with sulfate in the acid soil solution as well as in the root-cell vacuoles. In this review, we focus on recent insights into the mechanisms governing plant responses to Al toxicity and its relationship with sulfur nutrition, emphasizing the role of phytohormones, microRNAs, and ion transporters in higher plants. It is known that Al³⁺ disturbs gene expression and enzymes involved in biosynthesis of S-containing cysteine in root cells. On the other hand, Al³⁺ may induce ethylene biosynthesis, enhance reactive oxygen species production, alter phytohormone transport, trigger root growth inhibition and promote sulfate uptake under S deficiency. MicroRNA395, regulated by both Al toxicity and sulfate deprivation, represses its low-affinity Sulfate Transporter 2;1 (SULTR2;1) target. In addition, sulfate deprivation induces High Affinity Sulfate Transporters (HAST; SULTR1;2), improving sulfate uptake from low-sulfate soil solutions. Identification of new microRNAs and cloning of their target genes are necessary for a better understanding of the role of molecular regulation of plant resistance to Al stress and sulfate deprivation.

Molecular Mechanisms of Acclimatization to Phosphorus Starvation and Recovery Underlying Full-Length Transcriptome Profiling in Barley (Hordeum vulgare L.)

A lack of phosphorus (P) in plants can severely constrain growth and development. Barley, one of the earliest domesticated crops, is extensively planted in poor soil around the world. To date, the molecular mechanisms of enduring low phosphorus, at the transcriptional level, in barley are still unclear. In the present study, two different barley genotypes (GN121 and GN42)-with contrasting phosphorus efficiency-were used to reveal adaptations to low phosphorus stress, at three time points, at the morphological, physiological, biochemical, and transcriptome level. GN121 growth was less affected by phosphorus starvation and recovery than that of GN42. The biomass and inorganic phosphorus concentration of GN121 and GN42 declined under the low phosphorus-induced stress and increased after recovery with normal phosphorus. However, the range of these parameters was higher in GN42 than in GN121. Subsequently, a more complete genome annotation was obtained by correcting with the data sequenced on Illumina HiSeq X 10 and PacBio RSII SMRT platform. A total of 6,182 and 5,270 differentially expressed genes (DEGs) were identified in GN121 and GN42, respectively. The majority of these DEGs were involved in phosphorus metabolism such as phospholipid degradation, hydrolysis of phosphoric enzymes, sucrose synthesis, phosphorylation/dephosphorylation and post-transcriptional regulation; expression of these genes was significantly different between GN121 and GN42. Specifically, six and seven DEGs were annotated as phosphorus transporters in roots and leaves, respectively. Furthermore, a putative model was constructed relying on key metabolic pathways related to phosphorus to illustrate the higher phosphorus efficiency of GN121 compared to GN42 under low phosphorus conditions. Results from this study provide a multi-transcriptome database and candidate genes for further study on phosphorus use efficiency (PUE).

Long-distance ABA transport can mediate distal tissue responses by affecting local ABA concentrations

Environmental stresses that perturb plant water relations influence abscisic acid (ABA) concentrations, but it is unclear whether long-distance ABA transport contributes to changes in local ABA levels. To determine the physiological relevance of ABA transport, we made reciprocal- and self-grafts of ABA-deficient flacca mutant and wild-type (WT) tomato plants, in which low phosphorus (P) conditions decreased ABA concentrations while salinity increased ABA concentrations. Whereas foliar ABA concentrations in the WT scions were rootstock independent under conditions, salinity resulted in long-distance transport of ABA: flacca scions had approximately twice as much ABA when grafted on WT rootstocks compared to flacca rootstocks. Root ABA concentrations were scion dependent: both WT and flacca rootstocks had less ABA with the flacca mutant scion than with the WT scion under conditions. In WT scions, whereas rootstock genotype had limited effects on stomatal conductance under conditions, a flacca rootstock decreased leaf area of stressed plants, presumably due to attenuated root-to-shoot ABA transport. In flacca scions, a WT rootstock decreased stomatal conductance but increased leaf area of stressed plants, likely due to enhanced root-to-shoot ABA transport. Thus, long-distance ABA transport can affect responses in distal tissues by changing local ABA concentrations.

Dopamine alleviates nutrient deficiency-induced stress in Malus hupehensis

Dopamine mediates many physiological processes in plants. We investigated its role in regulating growth, root system architecture, nutrient uptake, and responses to nutrient deficiencies in Malus hupehensis Rehd. Under a nutrient deficiency, plants showed significant reductions in growth, chlorophyll concentrations, and net photosynthesis, along with disruptions in nutrient uptake, transport, and distribution. However, pretreatment with 100 μM dopamine markedly alleviated such inhibitions. Supplementation with that compound enabled plants to maintain their photosynthetic capacity and development of the root system while promoting the uptake of N, P, K, Ca, Mg, Fe, Mn, Cu, Zn, and B, altering the way in which those nutrients were partitioned throughout the plant. The addition of dopamine up-regulated genes for antioxidant enzymes involved in the ascorbate-glutathione cycle (MdcAPX, MdcGR, MdMDHAR, MdDHAR-1, and MdDHAR-2) but down-regulated genes for senescence (SAG12, PAO, and MdHXK). These results indicate that exogenous dopamine has an important antioxidant and anti-senescence effect that might be helpful for improving nutrient uptake. Our findings demonstrate that dopamine offers new opportunities for its use in agriculture, especially when addressing the problem of nutrient deficiencies.

The Understanding of the Plant Iron Deficiency Responses in Strategy I Plants and the Role of Ethylene in This Process by Omic Approaches

Iron (Fe) is an essential plant micronutrient but is toxic in excess. Fe deficiency chlorosis is a major constraint for plant growth and causes severe losses of crop yields and quality. Under Fe deficiency conditions, plants have developed sophisticated mechanisms to keep cellular Fe homeostasis via various physiological, morphological, metabolic, and gene expression changes to facilitate the availability of Fe. Ethylene has been found to be involved in the Fe deficiency responses of plants through pharmacological studies or by the use of ethylene mutants. However, how ethylene is involved in the regulations of Fe starvation responses remains not fully understood. Over the past decade, omics approaches, mainly focusing on the RNA and protein levels, have been used extensively to investigate global gene expression changes under Fe-limiting conditions, and thousands of genes have been found to be regulated by Fe status. Similarly, proteome profiles have uncovered several hallmark processes that help plants adapt to Fe shortage. To find out how ethylene participates in the Fe deficiency response and explore putatively novel regulators for further investigation, this review emphasizes the integration of those genes and proteins, derived from omics approaches, regulated both by Fe deficiency, and ethylene into a systemic network by gene co-expression analysis.

System analysis of metabolism and the transcriptome in Arabidopsis thaliana roots reveals differential co-regulation upon iron, sulfur and potassium deficiency

Deprivation of mineral nutrients causes significant retardation of plant growth. This retardation is associated with nutrient-specific and general stress-induced transcriptional responses. In this study, we adjusted the external supply of iron, potassium and sulfur to cause the same retardation of shoot growth. Nevertheless, limitation by individual nutrients resulted in specific morphological adaptations and distinct shifts within the root metabolite fingerprint. The metabolic shifts affected key metabolites of primary metabolism and the stress-related phytohormones, jasmonic, salicylic and abscisic acid. These phytohormone signatures contributed to specific nutrient deficiency-induced transcriptional regulation. Limitation by the micronutrient iron caused the strongest regulation and affected 18% of the root transcriptome. Only 130 genes were regulated by all nutrients. Specific co-regulation between the iron and sulfur metabolic routes upon iron or sulfur deficiency was observed. Interestingly, iron deficiency caused regulation of a different set of genes of the sulfur assimilation pathway compared with sulfur deficiency itself, which demonstrates the presence of specific signal-transduction systems for the cross-regulation of the pathways. Combined iron and sulfur starvation experiments demonstrated that a requirement for a specific nutrient can overrule this cross-regulation. The comparative metabolomics and transcriptomics approach used dissected general stress from nutrient-specific regulation in roots of Arabidopsis.

Root-to-Shoot Hormonal Communication in Contrasting Rootstocks Suggests an Important Role for the Ethylene Precursor Aminocyclopropane-1-carboxylic Acid in Mediating Plant Growth under Low-Potassium Nutrition in Tomato

Selection and breeding of rootstocks that can tolerate low K supply may increase crop productivity in low fertility soils and reduce fertilizer application. However, the underlying physiological traits are still largely unknown. In this study, 16 contrasting recombinant inbred lines (RILs) derived from a cross between domestic and wild tomato species (Solanum lycopersicum × Solanum pimpinellifolium) have been used to analyse traits related to the rootstock-mediated induction of low (L, low shoot fresh weight) or high (H, high shoot fresh weight) vigor to a commercial F1 hybrid grown under control (6 mM, c) and low-K (1 mM, k). Based on hormonal and ionomic composition in the root xylem sap and the leaf nutritional status after long-term (7 weeks) exposure low-K supply, a model can be proposed to explain the rootstocks effects on shoot performance with the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC) playing a pivotal negative role. The concentration of this hormone was higher in the low-vigor Lc and Lk rootstocks under both conditions, increased in the sensitive HcLk plants under low-K while it was reduced in the high-vigor Hk ones. Low ACC levels would promote the transport of K vs. Na in the vigorous Hk grafted plants. Along with K, Ca, and S, micronutrient uptake and transport were also activated in the tolerant Hk combinations under low-K. Additionally, an interconversion of trans-zeatin into trans-zeatin riboside would contribute to decrease ACC in the tolerant LcHk plants. The high vigor induced by the Hk plants can also be explained by an interaction of ACC with other hormones (cytokinins and salicylic, abscisic and jasmonic acids). Therefore, Hk rootstocks convert an elite tomato F1 cultivar into a (micro) nutrient-efficient phenotype, improving growth under reduced K fertilization.